To effectively guide behavior in a three-dimensional (3D) world, the visual system must create a 3D representation of one's surroundings. Each retinal image is simply a two- dimensional projection of 3D space, and thus contains no explicit depth information. The brain must therefore compute the 3D structure of a scene from the pair of retinal images. Binocular disparities between corresponding features in the two images provide precise, quantitative information about 3D scene structure, and these disparities are known to be encoded by neurons in primary visual cortex (V1). Recent studies show, however, that the representation of binocular disparity in V1 is not sufficient to account for stereoscopic depth perception. It is therefore critical to understand disparity processing in the extrastriate visual areas to which V1 projects. The goal of this research is to understand how disparity signals emanating from V1 are processed, transformed, and read out of extrastriate cortex to mediate stereoscopic depth perception. Our preliminary studies suggest that cortical area MT plays an important role in this process. The proposed experiments are designed to explore the nature of the disparity representation in MT, and to evaluate the contributions that MT makes to depth discrimination. There are four specific aims. The first aim is to determine whether MT neurons signal absolute or relative disparities. Relative disparities are critical for depth perception, but V1 neurons encode only absolute disparities. The second aim is to establish whether area MT contributes to perceptual discrimination of fine disparities. Single- and multi-unit responses will be recorded while monkeys perform a stereoacuity task, and electrical microstimulation will be used to probe for a causal link between neuronal activity and task performance. The third aim is to determine if MT neurons signal 3D surface orientation defined by gradients of horizontal disparity. The fourth aim is to test whether MT neurons combine horizontal disparity signals with an estimate of viewing distance (derived from vertical disparities) to encode egocentric distance and depth. By helping to elucidate the neurobiological basis of perception, these studies should ultimately lead to new approaches for treating diseases that impair visual cognition. This research should also be helpful for evaluating the design and safety of 3D virtual environments.